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I have encountered this reasoning quite frequently:

Somebody posits the hypothesis that an event X can happpen. A recent example I encountered was "vinegar and salt in the boiling water make eggs easier to peel afterwards", but I have seen many more. Another person imagines a way in which it could happen, judges that way to be impossible, and claims that X can never happen. In that example, the reply could be "salt and vinegar cannot cross the shell, so they will not make the egg easier to peel".

I have seen two ways in which this argument can be false. First, the replying person might just have wrong knowledge (maybe vinegar or salt can pass the shell?) - but I don't think this is a problem of the reasoning process, it is just correct reasoning based on a false assumption. What I find more interesting is that frequently, there can be other mechanisms for X to happen - maybe vinegar doesn't have to pass the shell to make the egg peelable? - but the person making the argument overlooks that possibility and goes from "the mechanism for X that I have in mind doesn't work" to the conclusion "X is completely impossible".

Another example would be people who insist that placebo does not help against pain, because per definition, it contains no chemically active substance that acts on pain receptors.

Has this flaw in reasoning received a recognizable name? I am asking specifically about fallacies as they apply to a formal reasoning process, not a general reason why arguing against vinegar in egg water would be a wrong position to take. (In fact, I don't even know if vinegar in egg water has any effect).

This fallacy is generally called argument from incredulity, or argument from failure of imagination. https://rationalwiki.org/wiki/Argument_from_incredulity It also is a subset of the argument from ignorance. Informal fallacies often overlap, and bleed into each other.

This fallacy is a common one,even among respected philosophers, as it is a characteristic of people that we are poor at identifying our own blindspots. And example of this mistake in a major philosopher is Jaegwon Kim's Pairing Problem argument, where point 3b and 4 is an argument from failure of imagination:

1. For substance dualism to be intelligible, the mental and the physical must be able to causally interact.


2. If there is causal interaction between any two events (say the
   mental act of the immaterial entity and the physical act of the material en
   tity), then there must be a pairing relation between them.


3. In all physical cases, the specific spatial relations between relata allow us
   to pick out the appropriate pairing relation, either by having direct spatial relations or by being connected by a spatially contiguous chain of events. There seems to be nothing else that can play this role.


4. Thus, it is highly likely pairing relations are only possible when both entities involved occupy specific spatial coordinates.


5. The events in mental-to-physical causation in substance dualism do not both occupy specific spatial coordinates. Thus, there seems to be no way to construct the appropriate causal pairing relation in these cases.


6. Thus, substance dualism is not intelligible.

What makes this a fallacy by Kim is because he does not test and support his assumption that location, and location alone, allows for pairing. For example, matter pairs, even though at the heisenberg scale -- matter is not fully localized. And even though matter is localized at the macro scale, it does not always pair -- as waves and fields can and do superimpose. Plus, with stream f consciousness, reasoning, and math, we have examples of apprently non-localized exclusions and causation (hence pairing). What all these examples show is that while location is often useful to achieve pairing, it is not always sufficient, nor the only way to do pairing.

Argument from incredulity (aka: appeal to common sense) :

I cannot imagine how P could be true; therefore P must be false.

Because a claim is made from ignorance (or even partial ignorance) this argument could be classified as an argument from ignorance.

However, not all arguments from ignorance are fallacious. Some may simply be weak arguments. Douglas Walton notes the following: (page 272)

In an inquiry, the argumentum ad ignorantiam for a particular proposition becomes stronger and stronger as the knowledge cumulated by the inquiry becomes more and more firmly established. However, even at the beginning stages of an inquiry, where very little is known about a subject or hypothesis, the argumentum ad ignorantiam can still be a correct, even if weak, kind of argument. It can be a speculative argument that only yields a small degree of plausibility for its conclusion. Even so, in such a case, it can be a correct, as opposed to an erroneous argument.

Where do such arguments go wrong? It is not just the presence of the argument but "its misemployment as a tactic to make an argument seem (unjustifiably) stronger than it really is." (page 274)

Locke's insight brought out this tactical element very well when he wrote (Hamblin, 1970,60) that men use the argumentum ad ignorantiam to "drive" others and "force" them to "submit" in debate.

Here is the question: Has this flaw in reasoning received a recognizable name? I am asking specifically about fallacies as they apply to a formal reasoning process...

An appropriate name would likely be "argument from ignorance". Such an argument might be weak. For it to be considered erroneous or a fallacy, however, requires a context where the one using the argument from ignorance is driving others to submit in debate. Just using the argument by itself is not enough for there to be fallacious reasoning.

Walton, D. (1996). Arguments from ignorance. Penn State Press.

The argument you discuss in the title is a form of argument from ignorance, but the example you give of

Another person imagines a way in which it could happen, judges that way to be impossible, and claims that X can never happen.

is a form of denying the antecedent: "If Y were true, then X could happen, but Y is not true, therefore X can't happen." Argument from ignorance and denying the antecedent are somewhat similar fallacies, and argument from ignorance could be considered to be a form of denying the antecedent: "If I knew of evidence that X is true, then I should accept X, but I don't know of such evidence, so I shouldn't accept X."